On TV they showed a little boy and his elder sister. The mother of the children left home. The boy was constantly rubbing both his eyes with both his fists. I was afraid he'd injure his eyes. They showed the children on TV again a couple of years later. The mother has not come back. The boy has reconciled to the fact that he must live without the mother. But his elder sister has now realized that she has to live without the mother and can't control her tears. My point is that your niece does not like the fact that your brother is living with his girl friend and not with her mother. He does not have to bring his girl friend home and pretend that she is his new wife. Eventually she may become his new wife. But he has to wait until the divorce. I believe it is your brother who is being immature and not your niece. I was in a court once. There was a girl about fifteen. Her parents were divorcing. She was a witness in the case. And she was crying constantly. But your brother is not even divorced yet and is bringing his gf home (or is he already divorced). You have to understand your niece. You have to be a good aunt even if she does not speak to your brother.